1. Field of the Invention
The present invention relates to a microscope, and more particularly to a click-stop structure provided on a rotary knob for continuously varying the magnification of a specimen observed under a microscope.
2. Related Background Art
Some of the conventionally known microscopes are provided with a variable-magnification optical system for varying the magnification of the specimen to be observed through the eyepiece lens of the microscope. Such microscopes are usually provided with a rotary knob for varying the magnification. Such microscopes will be further explained, by referring to a stereoscopic microscope shown in FIG. 4, which is an elevation view of said stereoscopic microscope. On a specimen stage 31 there is placed a specimen S to be observed. The image of the specimen S is observed by an eyepiece lens 34, through an objective 32, and a zoom lens system (variable-magnification optical system) composed of plural lenses provided in the main body (hereinafter called lens barrel) of the microscope. The observer can adjust the magnification of the specimen, observed through the eyepiece lens, by rotating a rotary knob 21, thereby moving at least one lens in the variable-magnification optical system in the axial direction of the objective optical system.
A rotary knob of this kind is illustrated in FIG. 3. The rotary knob 21 is fixed, by a screw 20, to a rotary shaft 2 which engages a magnification-varying cam 28. Thus, rotation of the rotary knob 21 vertically moves the magnification-varying lens 29, thereby continuously varying the magnification. Inside the rotary knob 21 there is provided a steel ball 5 which is pressed by a coil spring 4 in the axial direction of the rotary shaft 2 and fits in a stopping groove 25 provided in the lens barrel 1 of the microscope. Said stopping groove 25 is provided so as to correspond to the rotating position of the rotary knob when the variable-magnification optical system is in a specified magnification. More specifically, a stopping groove is provided for every magnification increase of 1 or 0.5 times. Thus a click-stop mechanism is constituted by the steel ball 5 and the stopping groove 25. On the external periphery 21a of the rotary knob 21 Close to the lens barrel 1, there is provided a magnification scale 26, indicating the image magnification of the variable-magnification optical system, and said scale is adjusted in advance so that the magnification of the variable-magnification optical system is indicated by an index mark 27 of the lens barrel 1.
The magnification varying operation has to be done cautiously by matching a desired value of the magnification scale 26 with the index mark 27 in magnifications other than the above-mentioned specified ones, but, in said specified magnifications, it is only required to match the magnification scale 26 roughly with the index mark 27 and to rotate the rotary knob until the above-mentioned click-stop mechanism functions.
However, the image magnification required by the observer is not necessarily limited to such specified ones, but often varies to various values. For example, in the case of photomicrographing, the magnification does not necessarily match one of the specified ones because the image frame of photographing is selected according to the size of the specimen. Particularly in the case of photographing plural specimens with a constant magnification, it is necessary to operate the rotary knob while watching the magnification scale so as to exactly reproduce the desired magnification or to effect photographing at a magnification, where the click-stop mechanism functions, close to the desired magnification. Such a situation is not satisfactory to the user.